<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <ul>
        <li id="elem">第一个节点</li>
        <li>第二个节点</li>
        <li>第三个节点</li>
        <li>第四个节点</li>
    </ul>
    </body>
    <script src="./jQuery2.js"></script>
    <script>

        console.log($("#elem") );
        console.log($("#elem").length );

        console.log($("ul li") );
        console.log($("ul li").length );

        console.log($("#elem2") );
        console.log($("#elem2").length );
    </script>
</html>